Data communication services using the Internet have made great strides and data communication by IP (Internet Protocol) packets is becoming the mainstream of traffic in the entire data communication.
In particular, a demand for services, such as speech or dynamic image data transmission, which urgently require to be provided on a real-time basis has risen markedly and the capacity of devices has increased markedly because of an increase in data communication speed.
As a result, devices for transmitting large-capacity data over a great distance are shifting to a packet transport system. With SONET (Synchronous Optical Network), SDH (Synchronous Digital Hierarchy), OTN (Optical Transport Network), or the like target data is mapped to frames to be continuously transmitted at a constant transmission rate, and is transmitted. At present there is a shift from SONET, SDH, OTN, or the like to the packet transport system. Accordingly, it is necessary to apply, for example, SONET, SDH, or OTN applications to transmission systems for the purpose of accommodating high speed and large capacity.
With transmission systems, on the other hand, switching devices are used for connecting networks. With an increase in the number of lines or the capacity of lines, the importance of switching devices is increasing and development of high-quality switching devices is expected.
Formerly a switching technique for inserting a plurality of subswitches between speech path stages to avoid link congestion caused by copy cells was proposed (see Japanese Laid-open Patent Publication No. 04-223692). In addition, a switching technique for reducing delay by placing a sequence control circuit for correcting cell sequence inversion was proposed (see Japanese Laid-open Patent Publication No. 05-268251).
With conventional switching devices, however, the following problem, for example, arises. When plural pieces of data transmitted to the same destination concentrate at switching time, data congestion (conflict) occurs. As a result, a transmission delay due to queuing occurs in transmitting data to the same destination.